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         mentation of kernel objects which perform various networking functions.
         The objects, known as nodes, can be arranged into arbitrarily complicated
         graphs.  Nodes have hooks which are used to connect two nodes together,
         forming the edges in the graph.  Nodes communicate along the edges to
         process data, implement protocols, etc.
         The aim of netgraph is to supplement rather than replace the existing
         kernel networking infrastructure.  It provides:
         ?   A flexible way of combining protocol and link level drivers.
         ?   A modular way to implement new protocols.
         ?   A common framework for kernel entities to inter-communicate.
         ?   A reasonably fast, kernel-based implementation.
       Nodes and Types
         The most fundamental concept in netgraph is that of a node.  All nodes
         implement a number of predefined methods which allow them to interact
         with other nodes in a well defined manner.
         Each node has a type, which is a static property of the node determined
         at node creation time.  A node's type is described by a unique ASCII type
         name.  The type implies what the node does and how it may be connected to
         other nodes.
         In object-oriented language, types are classes, and nodes are instances
         of their respective class.  All node types are subclasses of the generic
         node type, and hence inherit certain common functionality and capabili-
         ties (e.g., the ability to have an ASCII name).
         Nodes may be assigned a globally unique ASCII name which can be used to
         refer to the node.  The name must not contain the characters '.' or ':',
         and is limited to NG_NODESIZ characters (including the terminating NUL
         Each node instance has a unique ID number which is expressed as a 32-bit
         hexadecimal value.  This value may be used to refer to a node when there
         is no ASCII name assigned to it.
         Nodes are connected to other nodes by connecting a pair of hooks, one
         from each node.  Data flows bidirectionally between nodes along connected
         pairs of hooks.  A node may have as many hooks as it needs, and may
         assign whatever meaning it wants to a hook.
         Hooks have these properties:
         ?   A hook has an ASCII name which is unique among all hooks on that node
             (other hooks on other nodes may have the same name).  The name must
             not contain the characters '.' or ':', and is limited to NG_HOOKSIZ
             characters (including the terminating NUL character).
         ?   A hook is always connected to another hook.  That is, hooks are cre-
         connecting to the hook named debug might trigger the node to start send-
         ing debugging information to that hook.
       Data Flow
         Two types of information flow between nodes: data messages and control
         messages.  Data messages are passed in mbuf chains along the edges in the
         graph, one edge at a time.  The first mbuf in a chain must have the
         M_PKTHDR flag set.  Each node decides how to handle data received through
         one of its hooks.
         Along with data, nodes can also receive control messages.  There are
         generic and type-specific control messages.  Control messages have a com-
         mon header format, followed by type-specific data, and are binary struc-
         tures for efficiency.  However, node types may also support conversion of
         the type-specific data between binary and ASCII formats, for debugging
         and human interface purposes (see the NGM_ASCII2BINARY and
         NGM_BINARY2ASCII generic control messages below).  Nodes are not required
         to support these conversions.
         There are three ways to address a control message.  If there is a
         sequence of edges connecting the two nodes, the message may be "source
         routed" by specifying the corresponding sequence of ASCII hook names as
         the destination address for the message (relative addressing).  If the
         destination is adjacent to the source, then the source node may simply
         specify (as a pointer in the code) the hook across which the message
         should be sent.  Otherwise, the recipient node's global ASCII name (or
         equivalent ID-based name) is used as the destination address for the mes-
         sage (absolute addressing).  The two types of ASCII addressing may be
         combined, by specifying an absolute start node and a sequence of hooks.
         Only the ASCII addressing modes are available to control programs outside
         the kernel; use of direct pointers is limited to kernel modules.
         Messages often represent commands that are followed by a reply message in
         the reverse direction.  To facilitate this, the recipient of a control
         message is supplied with a "return address" that is suitable for address-
         ing a reply.
         Each control message contains a 32-bit value, called a "typecookie",
         indicating the type of the message, i.e. how to interpret it.  Typically
         each type defines a unique typecookie for the messages that it under-
         stands.  However, a node may choose to recognize and implement more than
         one type of messages.
         If a message is delivered to an address that implies that it arrived at
         that node through a particular hook (as opposed to having been directly
         addressed using its ID or global name) then that hook is identified to
         the receiving node.  This allows a message to be re-routed or passed on,
         should a node decide that this is required, in much the same way that
         data packets are passed around between nodes.  A set of standard messages
         for flow control and link management purposes are defined by the base
         system that are usually passed around in this manner.  Flow control mes-
         sage would usually travel in the opposite direction to the data to which
         implements a reader/writer semantic so that when there is a writer in the
         node, all other requests are queued, and while there are readers, a
         writer, and any following packets are queued.  In the case where there is
         no reason to queue the data, the input method is called directly, as men-
         tioned above.
         A node may declare that all requests should be considered as writers, or
         that requests coming in over a particular hook should be considered to be
         a writer, or even that packets leaving or entering across a particular
         hook should always be queued, rather than delivered directly (often use-
         ful for interrupt routines who want to get back to the hardware quickly).
         By default, all control message packets are considered to be writers
         unless specifically declared to be a reader in their definition.  (See
         NGM_READONLY in
         While this mode of operation results in good performance, it has a few
         implications for node developers:
         ?   Whenever a node delivers a data or control message, the node may need
             to allow for the possibility of receiving a returning message before
             the original delivery function call returns.
         ?   Netgraph provides internal synchronization between nodes.  Data
             always enters a "graph" at an edge node.  An edge node is a node that
             interfaces between netgraph and some other part of the system.  Exam-
             ples of "edge nodes" include device drivers, the socket, ether, tty,
             and ksocket node type.  In these edge nodes, the calling thread
             directly executes code in the node, and from that code calls upon the
             netgraph framework to deliver data across some edge in the graph.
             From an execution point of view, the calling thread will execute the
             netgraph framework methods, and if it can acquire a lock to do so,
             the input methods of the next node.  This continues until either the
             data is discarded or queued for some device or system entity, or the
             thread is unable to acquire a lock on the next node.  In that case,
             the data is queued for the node, and execution rewinds back to the
             original calling entity.  The queued data will be picked up and pro-
             cessed by either the current holder of the lock when they have com-
             pleted their operations, or by a special netgraph thread that is
             activated when there are such items queued.
         ?   It is possible for an infinite loop to occur if the graph contains
         So far, these issues have not proven problematical in practice.
       Interaction with Other Parts of the Kernel
         A node may have a hidden interaction with other components of the kernel
         outside of the netgraph subsystem, such as device hardware, kernel proto-
         col stacks, etc.  In fact, one of the benefits of netgraph is the ability
         to join disparate kernel networking entities together in a consistent
         communication framework.
         Creation of a new node
             The constructor for the type is called.  If creation of a new node is
             allowed, constructor method may allocate any special resources it
             needs.  For nodes that correspond to hardware, this is typically done
             during the device attach routine.  Often a global ASCII name corre-
             sponding to the device name is assigned here as well.
         Creation of a new hook
             The hook is created and tentatively linked to the node, and the node
             is told about the name that will be used to describe this hook.  The
             node sets up any special data structures it needs, or may reject the
             connection, based on the name of the hook.
         Successful connection of two hooks
             After both ends have accepted their hooks, and the links have been
             made, the nodes get a chance to find out who their peer is across the
             link, and can then decide to reject the connection.  Tear-down is
             automatic.  This is also the time at which a node may decide whether
             to set a particular hook (or its peer) into the queueing mode.
         Destruction of a hook
             The node is notified of a broken connection.  The node may consider
             some hooks to be critical to operation and others to be expendable:
             the disconnection of one hook may be an acceptable event while for
             another it may effect a total shutdown for the node.
         Preshutdown of a node
             This method is called before real shutdown, which is discussed below.
             While in this method, the node is fully operational and can send a
             "goodbye" message to its peers, or it can exclude itself from the
             chain and reconnect its peers together, like the ng_tee(4) node type
         Shutdown of a node
             This method allows a node to clean up and to ensure that any actions
             that need to be performed at this time are taken.  The method is
             called by the generic (i.e., superclass) node destructor which will
             get rid of the generic components of the node.  Some nodes (usually
             associated with a piece of hardware) may be persistent in that a
             shutdown breaks all edges and resets the node, but does not remove
             it.  In this case, the shutdown method should not free its resources,
             but rather, clean up and then call the NG_NODE_REVIVE() macro to sig-
             nal the generic code that the shutdown is aborted.  In the case where
             the shutdown is started by the node itself due to hardware removal or
             unloading (via ng_rmnode_self()), it should set the NGF_REALLY_DIE
             flag to signal to its own shutdown method that it is not to persist.
       Sending and Receiving Data
         Two other methods are also supported by all nodes:
         Receive data message
             A netgraph queueable request item, usually referred to as an item, is
             If it is only required to examine the contents of the mbufs, then it
             is possible to use the NGI_M() macro to both read and rewrite mbuf
             pointer inside the item.
             If developer needs to pass any meta information along with the mbuf
             chain, he should use mbuf_tags(9) framework.  Note that old netgraph
             specific meta-data format is obsoleted now.
             The receiving node may decide to defer the data by queueing it in the
             netgraph NETISR system (see below).  It achieves this by setting the
             HK_QUEUE flag in the flags word of the hook on which that data will
             arrive.  The infrastructure will respect that bit and queue the data
             for delivery at a later time, rather than deliver it directly.  A
             node may decide to set the bit on the peer node, so that its own out-
             put packets are queued.
             The node may elect to nominate a different receive data function for
             data received on a particular hook, to simplify coding.  It uses the
             NG_HOOK_SET_RCVDATA(hook, fn) macro to do this.  The function
             receives the same arguments in every way other than it will receive
             all (and only) packets from that hook.
         Receive control message
             This method is called when a control message is addressed to the
             node.  As with the received data, an item is received, with a pointer
             to the control message.  The message can be examined using the
             NGI_MSG() macro, or completely extracted from the item using the
             NGI_GET_MSG() which also removes the reference within the item.  If
             the Item still holds a reference to the message when it is freed
             (using the NG_FREE_ITEM() macro), then the message will also be freed
             appropriately.  If the reference has been removed, the node must free
             the message itself using the NG_FREE_MSG() macro.  A return address
             is always supplied, giving the address of the node that originated
             the message so a reply message can be sent anytime later.  The return
             address is retrieved from the item using the NGI_RETADDR() macro and
             is of type ng_ID_t.  All control messages and replies are allocated
             with the malloc(9) type M_NETGRAPH_MSG, however it is more convenient
             to use the NG_MKMESSAGE() and NG_MKRESPONSE() macros to allocate and
             fill out a message.  Messages must be freed using the NG_FREE_MSG()
             If the message was delivered via a specific hook, that hook will also
             be made known, which allows the use of such things as flow-control
             messages, and status change messages, where the node may want to for-
             ward the message out another hook to that on which it arrived.
             The node may elect to nominate a different receive message function
             for messages received on a particular hook, to simplify coding.  It
             uses the NG_HOOK_SET_RCVMSG(hook, fn) macro to do this.  The function
             receives the same arguments in every way other than it will receive
             all (and only) messages from that hook.
         Addresses are either absolute or relative.  An absolute address begins
         with a node name or ID, followed by a colon, followed by a sequence of
         hook names separated by periods.  This addresses the node reached by
         starting at the named node and following the specified sequence of hooks.
         A relative address includes only the sequence of hook names, implicitly
         starting hook traversal at the local node.
         There are a couple of special possibilities for the node name.  The name
         '.' (referred to as '.:') always refers to the local node.  Also, nodes
         that have no global name may be addressed by their ID numbers, by enclos-
         ing the hexadecimal representation of the ID number within the square
         brackets.  Here are some examples of valid netgraph addresses:
         The following set of nodes might be created for a site with a single
         physical frame relay line having two active logical DLCI channels, with
         RFC 1490 frames on DLCI 16 and PPP frames over DLCI 20:
         [type SYNC ]                  [type FRAME]                 [type RFC1490]
         [ "Frame1" ](uplink)<-->(data)[<un-named>](dlci16)<-->(mux)[<un-named>  ]
         [    A     ]                  [    B     ](dlci20)<---+    [     C      ]
                                                               |      [ type PPP ]
                                                                      [    D     ]
         One could always send a control message to node C from anywhere by using
         the name "Frame1:uplink.dlci16".  In this case, node C would also be
         notified that the message reached it via its hook mux.  Similarly,
         "Frame1:uplink.dlci20" could reliably be used to reach node D, and node A
         could refer to node B as ".:uplink", or simply "uplink".  Conversely, B
         can refer to A as "data".  The address "" could be used by both
         nodes C and D to address a message to node A.
         Note that this is only for control messages.  In each of these cases,
         where a relative addressing mode is used, the recipient is notified of
         the hook on which the message arrived, as well as the originating node.
         This allows the option of hop-by-hop distribution of messages and state
         information.  Data messages are only routed one hop at a time, by speci-
         fying the departing hook, with each node making the next routing deci-
         sion.  So when B receives a frame on hook data, it decodes the frame
         relay header to determine the DLCI, and then forwards the unwrapped frame
         to either C or D.
         In a similar way, flow control messages may be routed in the reverse
         direction to outgoing data.  For example a "buffer nearly full" message
         struct ng_node
             Node authors should always use the following typedef to declare their
             pointers, and should never actually declare the structure.
             typedef struct ng_node *node_p;
             The following properties are associated with a node, and can be
             accessed in the following manner:
                 A driver or interrupt routine may want to check whether the node
                 is still valid.  It is assumed that the caller holds a reference
                 on the node so it will not have been freed, however it may have
                 been disabled or otherwise shut down.  Using the
                 NG_NODE_IS_VALID(node) macro will return this state.  Eventually
                 it should be almost impossible for code to run in an invalid node
                 but at this time that work has not been completed.
             Node ID (ng_ID_t)
                 This property can be retrieved using the macro NG_NODE_ID(node).
             Node name
                 Optional globally unique name, NUL terminated string.  If there
                 is a value in here, it is the name of the node.
                       if (NG_NODE_NAME(node)[0] != '\0') ...
                       if (strcmp(NG_NODE_NAME(node), "fred") == 0) ...
             A node dependent opaque cookie
                 Anything of the pointer type can be placed here.  The macros
                 NG_NODE_SET_PRIVATE(node, value) and NG_NODE_PRIVATE(node) set
                 and retrieve this property, respectively.
             Number of hooks
                 The NG_NODE_NUMHOOKS(node) macro is used to retrieve this value.
                 The node may have a number of hooks.  A traversal method is pro-
                 vided to allow all the hooks to be tested for some condition.
                 NG_NODE_FOREACH_HOOK(node, fn, arg, rethook) where fn is a func-
                 tion that will be called for each hook with the form fn(hook,
                 arg) and returning 0 to terminate the search.  If the search is
                 terminated, then rethook will be set to the hook at which the
                 search was terminated.
         struct ng_hook
             Node authors should always use the following typedef to declare their
             hook pointers.
             typedef struct ng_hook *hook_p;
                 macro finds the peer.
                 The NG_HOOK_REF(hook) and NG_HOOK_UNREF(hook) macros increment
                 and decrement the hook reference count accordingly.  After decre-
                 ment you should always assume the hook has been freed unless you
                 have another reference still valid.
             Override receive functions
                 The NG_HOOK_SET_RCVDATA(hook, fn) and NG_HOOK_SET_RCVMSG(hook,
                 fn) macros can be used to set override methods that will be used
                 in preference to the generic receive data and receive message
                 functions.  To unset these, use the macros to set them to NULL.
                 They will only be used for data and messages received on the hook
                 on which they are set.
             The maintenance of the names, reference counts, and linked list of
             hooks for each node is handled automatically by the netgraph subsys-
             tem.  Typically a node's private info contains a back-pointer to the
             node or hook structure, which counts as a new reference that must be
             included in the reference count for the node.  When the node con-
             structor is called, there is already a reference for this calculated
             in, so that when the node is destroyed, it should remember to do a
             NG_NODE_UNREF() on the node.
             From a hook you can obtain the corresponding node, and from a node,
             it is possible to traverse all the active hooks.
             A current example of how to define a node can always be seen in
             src/sys/netgraph/ng_sample.c and should be used as a starting point
             for new node writers.
       Netgraph Message Structure
         Control messages have the following structure:
         #define NG_CMDSTRSIZ    32      /* Max command string (including nul) */
         struct ng_mesg {
           struct ng_msghdr {
             u_char      version;        /* Must equal NG_VERSION */
             u_char      spare;          /* Pad to 2 bytes */
             u_short     arglen;         /* Length of cmd/resp data */
             u_long      flags;          /* Message status flags */
             u_long      token;          /* Reply should have the same token */
             u_long      typecookie;     /* Node type understanding this message */
             u_long      cmd;            /* Command identifier */
             u_char      cmdstr[NG_CMDSTRSIZ]; /* Cmd string (for debug) */
           } header;
           char  data[0];                /* Start of cmd/resp data */
         #define NG_ABI_VERSION  5               /* Netgraph kernel ABI version */
         flags   Indicates whether this is a command or a response control mes-
         token   The token is a means by which a sender can match a reply message
                 to the corresponding command message; the reply always has the
                 same token.
                 The corresponding node type's unique 32-bit value.  If a node
                 does not recognize the type cookie it must reject the message by
                 returning EINVAL.
                 Each type should have an include file that defines the commands,
                 argument format, and cookie for its own messages.  The typecookie
                 insures that the same header file was included by both sender and
                 receiver; when an incompatible change in the header file is made,
                 the typecookie must be changed.  The de-facto method for generat-
                 ing unique type cookies is to take the seconds from the Epoch at
                 the time the header file is written (i.e., the output of "date -u
                 There is a predefined typecookie NGM_GENERIC_COOKIE for the
                 generic node type, and a corresponding set of generic messages
                 which all nodes understand.  The handling of these messages is
         cmd     The identifier for the message command.  This is type specific,
                 and is defined in the same header file as the typecookie.
         cmdstr  Room for a short human readable version of command (for debugging
                 purposes only).
         Some modules may choose to implement messages from more than one of the
         header files and thus recognize more than one type cookie.
       Control Message ASCII Form
         Control messages are in binary format for efficiency.  However, for
         debugging and human interface purposes, and if the node type supports it,
         control messages may be converted to and from an equivalent ASCII form.
         The ASCII form is similar to the binary form, with two exceptions:
         1.   The cmdstr header field must contain the ASCII name of the command,
              corresponding to the cmd header field.
         2.   The arguments field contains a NUL-terminated ASCII string version
              of the message arguments.
         In general, the arguments field of a control message can be any arbitrary
         C data type.  Netgraph includes parsing routines to support some pre-
         defined datatypes in ASCII with this simple syntax:
         ?   Integer types are represented by base 8, 10, or 16 numbers.
         ?   Any array element or structure field whose value is equal to its
             "default value" may be omitted.  For integer types, the default value
             is usually zero; for string types, the empty string.
         ?   Array elements and structure fields may be specified in any order.
         Each node type may define its own arbitrary types by providing the neces-
         sary routines to parse and unparse.  ASCII forms defined for a specific
         node type are documented in the corresponding man page.
       Generic Control Messages
         There are a number of standard predefined messages that will work for any
         node, as they are supported directly by the framework itself.  These are
         defined in #include <netgraph/ng_message.h>
         along with the basic layout of messages and other similar information.
                 Connect to another node, using the supplied hook names on either
                 Construct a node of the given type and then connect to it using
                 the supplied hook names.
                 The target node should disconnect from all its neighbours and
                 shut down.  Persistent nodes such as those representing physical
                 hardware might not disappear from the node namespace, but only
                 reset themselves.  The node must disconnect all of its hooks.
                 This may result in neighbors shutting themselves down, and possi-
                 bly a cascading shutdown of the entire connected graph.
                 Assign a name to a node.  Nodes can exist without having a name,
                 and this is the default for nodes created using the NGM_MKPEER
                 method.  Such nodes can only be addressed relatively or by their
                 ID number.
                 Ask the node to break a hook connection to one of its neighbours.
                 Both nodes will have their "disconnect" method invoked.  Either
                 node may elect to totally shut down as a result.
                 Asks the target node to describe itself.  The four returned
                 fields are the node name (if named), the node type, the node ID
                 and the number of hooks attached.  The ID is an internal number
                 unique to that node.
                 This returns the information given by NGM_NODEINFO, but in addi-
                 The node may return a text formatted status message.  The status
                 information is determined entirely by the node type.  It is the
                 only "generic" message that requires any support within the node
                 itself and as such the node may elect to not support this mes-
                 sage.  The text response must be less than NG_TEXTRESPONSE bytes
                 in length (presently 1024).  This can be used to return general
                 status information in human readable form.
                 This message converts a binary control message to its ASCII form.
                 The entire control message to be converted is contained within
                 the arguments field of the NGM_BINARY2ASCII message itself.  If
                 successful, the reply will contain the same control message in
                 ASCII form.  A node will typically only know how to translate
                 messages that it itself understands, so the target node of the
                 NGM_BINARY2ASCII is often the same node that would actually
                 receive that message.
                 The opposite of NGM_BINARY2ASCII.  The entire control message to
                 be converted, in ASCII form, is contained in the arguments sec-
                 tion of the NGM_ASCII2BINARY and need only have the flags,
                 cmdstr, and arglen header fields filled in, plus the
                 NUL-terminated string version of the arguments in the arguments
                 field.  If successful, the reply contains the binary version of
                 the control message.
       Flow Control Messages
         In addition to the control messages that affect nodes with respect to the
         graph, there are also a number of flow control messages defined.  At
         present these are not handled automatically by the system, so nodes need
         to handle them if they are going to be used in a graph utilising flow
         control, and will be in the likely path of these messages.  The default
         action of a node that does not understand these messages should be to
         pass them onto the next node.  Hopefully some helper functions will
         assist in this eventually.  These messages are also defined in #include
         and have a separate cookie NG_FLOW_COOKIE to help identify them.  They
         will not be covered in depth here.


         The base netgraph code may either be statically compiled into the kernel
         or else loaded dynamically as a KLD via kldload(8).  In the former case,
               options NETGRAPH
         in your kernel configuration file.  You may also include selected node
         types in the kernel compilation, for example:
         by calling ng_newtype(), supplying a pointer to the type's struct ng_type
         The NETGRAPH_INIT() macro automates this process by using a linker set.


         Several node types currently exist.  Each is fully documented in its own
         man page:
         SOCKET  The socket type implements two new sockets in the new protocol
                 domain PF_NETGRAPH.  The new sockets protocols are NG_DATA and
                 NG_CONTROL, both of type SOCK_DGRAM.  Typically one of each is
                 associated with a socket node.  When both sockets have closed,
                 the node will shut down.  The NG_DATA socket is used for sending
                 and receiving data, while the NG_CONTROL socket is used for send-
                 ing and receiving control messages.  Data and control messages
                 are passed using the sendto(2) and recvfrom(2) system calls,
                 using a struct sockaddr_ng socket address.
         HOLE    Responds only to generic messages and is a "black hole" for data.
                 Useful for testing.  Always accepts new hooks.
         ECHO    Responds only to generic messages and always echoes data back
                 through the hook from which it arrived.  Returns any non-generic
                 messages as their own response.  Useful for testing.  Always
                 accepts new hooks.
         TEE     This node is useful for "snooping".  It has 4 hooks: left, right,
                 left2right, and right2left.  Data entering from the right is
                 passed to the left and duplicated on right2left, and data enter-
                 ing from the left is passed to the right and duplicated on
                 left2right.  Data entering from left2right is sent to the right
                 and data from right2left to left.
         RFC1490 MUX
                 Encapsulates/de-encapsulates frames encoded according to RFC
                 1490.  Has a hook for the encapsulated packets (downstream) and
                 one hook for each protocol (i.e., IP, PPP, etc.).
                 Encapsulates/de-encapsulates Frame Relay frames.  Has a hook for
                 the encapsulated packets (downstream) and one hook for each DLCI.
                 Automatically handles frame relay "LMI" (link management inter-
                 face) operations and packets.  Automatically probes and detects
                 which of several LMI standards is in use at the exchange.
         TTY     This node is also a line discipline.  It simply converts between
                 mbuf frames and sequential serial data, allowing a TTY to appear
                 as a netgraph node.  It has a programmable "hotkey" character.
                 "ng0", "ng1", etc.
                 This node implements a simple round-robin multiplexer.  It can be
                 used for example to make several LAN ports act together to get a
                 higher speed link between two machines.
         Various PPP related nodes
                 There is a full multilink PPP implementation that runs in
                 netgraph.  The net/mpd port can use these modules to make a very
                 low latency high capacity PPP system.  It also supports PPTP VPNs
                 using the PPTP node.
         PPPOE   A server and client side implementation of PPPoE.  Used in con-
                 junction with either ppp(8) or the net/mpd port.
         BRIDGE  This node, together with the Ethernet nodes, allows a very flexi-
                 ble bridging system to be implemented.
                 This intriguing node looks like a socket to the system but
                 diverts all data to and from the netgraph system for further pro-
                 cessing.  This allows such things as UDP tunnels to be almost
                 trivially implemented from the command line.
         Refer to the section at the end of this man page for more nodes types.


         Whether a named node exists can be checked by trying to send a control
         message to it (e.g., NGM_NODEINFO).  If it does not exist, ENOENT will be
         All data messages are mbuf chains with the M_PKTHDR flag set.
         Nodes are responsible for freeing what they allocate.  There are three
         1.   Mbufs sent across a data link are never to be freed by the sender.
              In the case of error, they should be considered freed.
         2.   Messages sent using one of NG_SEND_MSG_*() family macros are freed
              by the recipient.  As in the case above, the addresses associated
              with the message are freed by whatever allocated them so the recipi-
              ent should copy them if it wants to keep that information.
         3.   Both control messages and data are delivered and queued with a
              netgraph item.  The item must be freed using NG_FREE_ITEM(item) or
              passed on to another node.


                 Definitions for use solely within the kernel by netgraph nodes.
                 Skeleton netgraph node.  Use this as a starting point for new
                 node types.


         There is a library for supporting user-mode programs that wish to inter-
         act with the netgraph system.  See netgraph(3) for details.
         Two user-mode support programs, ngctl(8) and nghook(8), are available to
         assist manual configuration and debugging.
         There are a few useful techniques for debugging new node types.  First,
         implementing new node types in user-mode first makes debugging easier.
         The tee node type is also useful for debugging, especially in conjunction
         with ngctl(8) and nghook(8).
         Also look in /usr/share/examples/netgraph for solutions to several common
         networking problems, solved using netgraph.


         socket(2), netgraph(3), ng_async(4), ng_atm(4), ng_atmllc(4),
         ng_bluetooth(4), ng_bpf(4), ng_bridge(4), ng_bt3c(4), ng_btsocket(4),
         ng_cisco(4), ng_device(4), ng_echo(4), ng_eiface(4), ng_etf(4),
         ng_ether(4), ng_fec(4), ng_frame_relay(4), ng_gif(4), ng_gif_demux(4),
         ng_h4(4), ng_hci(4), ng_hole(4), ng_hub(4), ng_iface(4), ng_ip_input(4),
         ng_ksocket(4), ng_l2cap(4), ng_l2tp(4), ng_lmi(4), ng_mppc(4),
         ng_netflow(4), ng_one2many(4), ng_ppp(4), ng_pppoe(4), ng_pptpgre(4),
         ng_rfc1490(4), ng_socket(4), ng_split(4), ng_sppp(4), ng_sscfu(4),
         ng_sscop(4), ng_tee(4), ng_tty(4), ng_ubt(4), ng_UI(4), ng_uni(4),
         ng_vjc(4), ng_vlan(4), ngctl(8), nghook(8)


         The netgraph system was designed and first implemented at Whistle Commu-
         nications, Inc. in a version of FreeBSD 2.2 customized for the Whistle
         InterJet.  It first made its debut in the main tree in FreeBSD 3.4.


         Julian Elischer <>, with contributions by Archie Cobbs

    BSD May 25, 2008 BSD


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